Bedside tip: How to use the expiratory time constant

The expiratory time constant (RCEXP) is a dynamic measurement of respiratory mechanics measured breath-by-breath on all Hamilton Medical ventilators.

RCEXP is reliable in both passive and spontaneously breathing patients, assuming expiration is passive. It may also be used during NIV, provided there are no unintentional leaks.

As RCEXP is the product of compliance and resistance, this single variable can provide us with an assessment of the overall respiratory mechanics. It is very useful for diagnosing the lung condition and its severity, optimizing the ventilator settings, monitoring prone positioning, and understanding certain respiratory events.

Expiratory time constant for diagnosing the lung condition

In a mechanically ventilated patient with a normal lung, RCEXP is normally between 0.5 and 0.7 s. However, it is important to check that compliance and resistance values are also within a normal range, because a mixed lung condition combining decreased compliance and increased resistance may result in a pseudo-normal RCEXP.

An expiratory time constant shorter than 0.5 s indicates a decrease in compliance, either due to the lung or the chest wall. In ARDS patients, RCEXP is typically in the range of 0.4 to 0.6 s. It is shorter in patients with more severe ARDS, indicating low compliance and a small volume of aerated lung. In patients with lung fibrosis or chest-wall stiffness such as kyphoscoliosis, RCEXP is usually very short and ranges from 0.15 to 0.25 s.

Typical respiratory mechanics monitoring for an ARDS patient

An expiratory time constant of longer than 0.7 s indicates increased resistance, which may be associated with increased compliance in the case of COPD patients with lung emphysema. A long RCEXP is typical in COPD and asthmatic patients. In patients with severe bronchospasm, RCEXP can be as long as 3 s. If the patient is not COPD or asthmatic, a long RCEXP may indicate incorrect positioning or kinking of the endotracheal tube.

Typical respiratory mechanics monitoring for a COPD patient

Expiratory time constant for optimizing ventilator settings

Patients with a short RCEXP are at risk of ventilator-induced lung injuries and should be closely monitored for tidal volume, driving pressure, and plateau pressure. In contrast, patients with a long RCEXP are at risk of dynamic hyperinflation, so intrinsic PEEP should be measured regularly.

In pressure-support and ASV® modes, expiratory trigger sensitivity (ETS) is an important setting for optimizing patient-ventilator synchronization. ETS represents the percentage of the maximum inspiratory flow at which the mechanical breath ends. A high percentage results in a short mechanical breath and vice versa. This setting can be optimized according to the respiratory mechanics. As an initial approach, ETS may be adjusted based on RCEXP as follows:

Normal RCEXP

ETS = 25%–40%

Short RCEXP

ETS = 5%–25%

Long RCEXP

ETS = 40%–70%

Expiratory time constant for monitoring prone positioning

The effect of prone positioning on respiratory mechanics can be assessed using the trend of RCEXP and compliance. If prone positioning is associated with lung recruitment, it is indicated by an increase in compliance and RCEXP. If RCEXP increases with no change to compliance, the clinician should check for incorrect positioning or kinking of the endotracheal tube.

The image below displays an example of respiratory mechanics trends in the supine and prone position. The cursor indicates the start of the prone-position session. After prone positioning, RCEXP and compliance both increase, indicating lung recruitment.

Respiratory mechanics trends in the supine and prone position

Expiratory time constant for understanding respiratory events

A sudden event leading to desaturation and/or an increase in airway pressure needs a rapid diagnosis. Looking at trends for RCEXP will help us to understand whether the event is related to a rapid change in respiratory mechanics. An increase in RCEXP indicates one of the following: an endotracheal tube obstruction or malpositioning, a patient biting the endotracheal tube, an excess of secretions, or a bronchospasm. Conversely, a decrease in RCEXP indicates a pneumothorax, pleural effusion, or atelectasis. Sudden desaturation without a change in RCEXP indicates a drop in cardiac output or a severe pulmonary embolism.

Related Articles

Date of Printing: 25.05.2019
Disclaimer:
The content of this newsletter is for informational purposes only and is not intended to be a substitute for professional training or for standard treatment guidelines in your facility. Any recommendations made in this newsletter with respect to clinical practice or the use of specific products, technology or therapies represent the personal opinion of the author only, and may not be considered as official recommendations made by Hamilton Medical AG. Hamilton Medical AG provides no warranty with respect to the information contained in this newsletter and reliance on any part of this information is solely at your own risk.

Cookie settings

We use cookies to personalize content and ads, to provide social media features and to analyze our website traffic. We also share information about your use of our site with our social media, advertising and analytics partners.
Because we respect your right to privacy, you can choose not to allow some types of cookies. Click here to find out more about our cookie policy.

Strictly necessary cookies

These cookies are necessary for the website to function and cannot be switched off in our systems. They are usually set only in response to actions made by you, such as setting your privacy preferences, logging in or filling in forms.

Google Analytics

Google Analytics provides us with statistics on how our website is used. All data collected is anonymous.

ClickDimensions

ClickDimensions provides us with information on how you use our website. This allows us to improve and personalize your experience on the site.

Google AdWords

The Google ad publishing and distribution network uses cookies to build a profile of your interests and show you relevant advertisements on other sites.

Facebook

The Facebook social networking service collects data through a range of plug-ins and integrations, which is used primarily for Facebook's own tracking and targeting.